Table of contents

A critical examination of some theoretical and experimental results on doubly excited resonances in helium has been made. It is concluded that the partial widths of the Rydberg series should vary approximately as n^-3.254, in variance with previous inferences. This estimate appears to be supported by semiclassical calculations of doubly excited two-electron systems, which give analytically Gamma approximately n^-3.

The authors report distorted-wave Born approximation (DWBA) calculations of the triple differential cross section for the process e^-+He(1s²) to e^-+e^-+He⁺(1s) in energy-sharing perpendicular plane geometry at incident energies from 34.6 to 200 eV. The cross section is given as a function of the angle phi between the two outgoing electrons. Their results reproduce well the pattern of corresponding experimental measurements of Woolf et al. (1989, 1990). A simple model analysis indicates that there should be a 'single scattering' peak at phi =180' and a 'double scattering' peak at phi =90'. In the double scattering mechanism the incident electron is first elastically scattered through 90' by the atom before ionising the target electron. At low energies single scattering should be dominant with double scattering taking over as the principal mechanism at high energies. Both DWBA and experiment confirm these predictions.

Laser-assisted excitation of the helium 1¹S to 2³S transition is studied in the threshold region using circularly polarised 10.6 mu m CO₂ radiation.

The authors demonstrate the practicability of a method for analysing coloured noise systems by an application to the F-spin fluctuation model of free induction decay. The accuracy of the Bourret and a previously employed cumulant-like approximation is assessed. The results of calculations are also compared with experimental data.

An elementary introduction on direct and resonant low-energy (0-20 eV) electron scattering from molecules condensed on surfaces is given with reference to concepts developed from gas-phase investigations. It is shown that while elastic scattering can be described in terms of band-structure parameters, inelastic processes are often governed by intramolecular mechanisms. Following a brief description of the experimental techniques, elastic scattering and resonant vibrational excitation of physisorbed and chemisorbed molecules are reviewed. The results of electron impact experiments leading to stable anion formation are also reviewed. Typical data are presented to illustrate the main characteristics of the processes involved in electron scattering from molecules condensed in submonolayer, single and multilayer coverages of various solid surfaces.

For problems expressible in the mathematical form of the one-electron two-fixed-centre Schrodinger equation, a recursive process, well adapted to microcomputers, is presented to obtain the expansion coefficients of the angular parts of the wavefunctions in terms of associated Legendre polynomials or spherical harmonics as well as their successive derivatives with respect to the internuclear distance.

Photoion spectra of Kr near the 3d ionisation limits have been measured using a threshold electron-photoion coincidence technique. The spectra of Kr²⁺ and Kr³⁺ ions show structures corresponding to the 3d^-1np Rydberg states below the ionisation limits and to 3d_5/2 and 3d_3/2 ionisation above the ionisation limits. It is proposed that the lower Rydberg states can decay by emitting threshold electrons through two-step autoionisation, such as Kr 3d^-15p to Kr⁺ 4p^-2(¹S)6p to Kr²⁺ 4p^-2(³P) and Kr 3d^-15p to Kr²⁺ 4s^-14p^-2np (n>or=5) to Kr³⁺ 4p^-3. The structures due to 3d threshold ionisation show the effect of post-collision interaction, namely peak shifts and asymmetric profiles. The shifts are almost the same in the Kr²⁺ and Kr³⁺ spectra.

For pt.III, see ibid., vol.21, no.10, p.1845, (1988). Near-threshold collision-induced dissociation is simulated numerically for the process Xe+Xe₂ to 3Xe by carrying out detailed classical trajectory calculations. A static model of the target is chosen and the system is assumed to have zero total angular momentum. Then by varying a single parameter, the threshold law sigma approximately E^1.6 for the cross section is derived, and this is consistent with analytical results obtained previously. The authors also consider the distributions of energy, mutual angles and individual angular momenta in the final channel.

The present study incorporates the nonlocal optical potentials for the continuum within the coupled-channels optical framework to study electron scattering from atomic hydrogen at 54.42 eV. Nine-state coupled-channels calculations with nonlocal and local continuum optical potentials were performed. The results for differential, total and ionisation cross sections as well as the 2p angular correlation parameters lambda and R are comparable with other nonperturbative calculations. There are still discrepancies between theory and experiment, particularly for lambda and R at larger angles.

The optical excitation function and the polarisation of the Ca II lambda =393.3 nm (4²P_3/4 to 4²S_1/2) line following simultaneous ionisation and excitation from Ca I ground-state atoms have been measured using crossed electron and calcium atom beams. The absolute excitation cross section of the Ca II ²P_3/2 state has been determined to be (0.72+or-0.27)*10^-16 cm² at an electron energy of 40 eV. The linear polarisation of the Ca II lambda =393.3 nm line has been measured from the 4²P_3/2 threshold to 200 eV. The polarisation was found to give a peak reading of 17% near threshold and to converge to zero at high energies.

It has been suggested that the reaction d+d to ³He+n has been observed at room temperature in electrolytically deuterated Ti and Pd. The reported neutron production rates cannot be explained on conventional grounds, and the possibility that neutrons are produced by fusion reactions in a small nonthermal population of fast deuterons has been raised. The K and L X-ray production which should result from these putative fast deuterons is calculated, and it is concluded that neutron production is a more sensitive probe of fast deuterons than X-rays. Estimates of the fusion rate/fast deuteron, as a function of deuteron energy are also made.